This invention relates to an angle-modulating communication system for transmitting radio frequency carrier waves which have been angle-modulated with frequency-division-multiplex signals, and more particularly to a system for suppressing those sideband components of angle-modulated multiplex signals which are to be branched out at a repeater station and removed from further relay.
It has been a general practice to use a system, in which radio carrier waves are angle-modulated with frequency division multiplex signals for radio transmission. In long-distance transmission employing the aforesaid system, there arises a demand that a part of the multiplex signals be branched out at intermediate radio repeater stations and a new signal be inserted, i.e., the so-called drop-and-insert facilities must be provided. To meet this demand, there has been proposed a detection relay system remodulation relay system. More specifically, the angle-modulated wave is demodulated to obtain frequency division multiplex baseband signals, and the signals are branched out and inserted at this stage and thereafter a carrier wave is angle-modulated for transmitting signals to the next repeater station. According to this system, frequency components having no bearing on the drop-and-insert process also tend to be demodulated and modulated, so that there will be incurred noise or distortion due to the modulator and demodulator. In addition, complex arrangements of modulating and demodulating circuits result, with a resulting increase in cost.
In contrast thereto, there has been proposed a branching system which has been termed a leak-off technique. With this technique, a part of radio signals leaks at a repeater station and is demodulated to take out desired frequency components alone. This system presents a simple device for branching signals, although there are encountered disadvantages, in that even after branching, such frequency components remain in the transmitting multiplex signals, and hence the aforesaid frequency components may be no longer used for the communication between repeater stations. This lowers the efficiency of frequency band use.
U.S. Pat. No. 3,294,914 patened on Dec. 27, 1966 in the name of Martin Muller is proposed to avoid these disadvantages. This patent teaches that after a desired signal has been branched according to the leak-off technique, the branched signal is phase-inverted to thereby frequency-modulate a local oscillating signal, so that an unwanted signal of the transmitting multiplex signals may be cancelled by this modulated signal. This attempt removes an unwanted spectrum from the transmitting signal, but frequency-modulates the local oscillating signal being delivered to the modulator, through which the transmitting signal passes, thus resulting in a complicated arrangement.